Anaerobic digestion is an energy conversion process that can produce biogas from the solid matter in sludge and reduce waste discharge. Biogas may be used for energy demands in an overall sludge processing or wastewater treatment system or in other areas. Digestion of sludge anaerobically occurs due to organisms present in the sludge and is known to occur in at least two general temperature regimes. At temperatures of about 32°-38° C., mesophilic organisms are active and contribute to digestion, while at temperatures of 50°-60° C. thermophilic organisms function to digest the sludge. Depending on the type of sludge being processed different population profiles of organisms may be involved, and anaerobic digestion is generally operated in a range consistent with the type of sludge and the organism profile in the sludge. Typically, it is desired to operate the anaerobic digester in the range of 30°-60° C. In order to compensate for heat loss from the digester and to assure that the operating temperature in the digester remains in the desired range, it is known to supply in feed sludge to the digester at a temperature that is approximately 1-10° C. above the desired anaerobic digester operation temperature. Also, an optimal dry solids concentration for anaerobic digestion of sludge is about 3%-10% dry solids concentration.
In preparing raw sludge for anaerobic digestion, it is known to subject the sludge to hydrolysis. Hydrolysis increases the biodegradability of the organic matter in the sludge, which increases biogas production and reduces waste output from anaerobic digestion. Hydrolysis of sludge takes place in temperature and pressure ranges of 150°-170° C. and 6-12 bar, respectively. It is preferable to dewater sludge to about 20%-30% by weight dry solids before feeding the sludge into a hydrolysis reactor in order to minimize reactor volume and energy consumption.
Temperature and pressure in the desired ranges for hydrolysis of sludge is typically created by injecting steam with the sludge into the hydrolysis reactor. Steam injection is a highly energy intensive aspect of sludge hydrolysis and waste energy recovery is a matter of significant concern relative to sludge hydrolysis. In some instances, it is known to partly recover energy by flashing off the steam into the incoming sludge in a batch hydrolysis process or by heating boiler feed water for the production of new steam. These processes are characterized by less than desirable efficiency and very high cost.
There continues to be a need to improve the energy efficiency of sludge treatment systems that include processes for hydrolyzing sludge and thereafter anaerobically digesting the sludge.